Posted 16 February 2009 - 02:23 PM
Already been applying this tag to a couple of clients where it was appropriate.
The first is a fairly predictable use for most SEOs dealing with larger-scale ecommerce clients and wider aspects of internet marketing cross-over campaigns. The client uses a powerful and robust CMS that is also a stock-management solution, ties into all their business, accounting, order management, everything. I mean everything, from contact management to stock management, to ordering management, to shipping and accounting. It is not bespoke, and it isn't as optimised for search performance as we'd like.
The system is shared by several other famous high street names, and only our client is pushing for SEO modifications. That's not great, obviously, but the overall business functionality of the site is way beyond anything else much I've seen out there that is any better optimised. Moving away from this CMS would be akin to severing a limb or two - only applicable to save a life, and not otherwise conceivable.
The company behind the CMS have been very cooperative overall, but they really seem to struggle with some concepts, and anyway, this isn't a simple piece of software, and changes to the code take a while and need to be made very carefully to avoid complications and knock-on effects.
This client of ours runs a couple of affiliate programs through third-parties such as Affiliate Window, and these affiliate programs are managed by another specialist agency. Affiliate links to the site go through the affiliate company tracking and redirect, so are not entirely sure to pass any PageRank anyway, but even so, they eventually land the visitor at the site via a URL something like www.example.com/default.htm?src=awin
The classic advice was always to have the server detect such a URL request, and serve a cookie with the referrer info, and redirect to a cleaner, canonical URL without a referrer in, like www.example.com
But of course, not everyone has cookie support enabled. So the classic solution of serving a cookie and a redirect can fail to track those users. In some sites, I've seen anything upto 15% of visitors have disabled cookies. So, that throws the whole tracking off, and can make the client's affiliate program a lot less valuable and desirable to potential affiliates - most especially those who know the most and are the best performers.
Using the Canonical Link tag (i.e. the Link tag with a relationship attribute set to specify the preferred canonical version of multiform URL - lets not start calling it a Canonical Tag, its only an attribute use of an existing LINK tag) offers an alternate solution. Let the referrer parameter remain in the URL, and use the ability to specify the correct canonical version prevent indexing, and pass the link value if any.
Personally, I'd still choose a 301 redirect and cookies in most cases, because otherwise visitors who arrive from an affiliate link may copy and paste the URL elsewhere, causing you to pay an affiliate more than you really needed to, but hey, it is the client's choice to make. My job is simply to present best practice guidance and good solid options.
Case 2 is a little less predictable for now, but you'll see it more and more once a few people catch on. Lots of sites offer a range of ways to view a list of products within a category. There's the most common such as listed by price (high to low and low to high), by popularity (most reviewed, highest rated, best selling), maybe with variable amounts of products per page (15, 25, 50, 100). You might also have viewing options such as with or without ratings, or with or without any other kind of funky added thing. I have personally seen sites that had scores of different ways of looking at the same list of products, and in some cases that included only image changes.
I can't use a 301 redirect for this because I want browsers to have all those view options. I might have spent months on thinking up every viewing option I could to enhance the personalisation and personal-usability of the site. 301 redirecting everyone back to the vanilla view is not an option. Here, the Canonical Link tag is about the only solution other than simply trying to weight the PR, or losing links in a complex set of NOFOLLOW link-pop sculpting.
This new ability to specify and suggest the correct canonical version of a page to index, and still carry all the link pop from any alternate versions is spot-on perfect for this second use.
There are many cases where the use of the Canonical Link tag would be slapping a band-aid on a broken leg, but there are also cases where using a 301 would be like applying a splint and plaster-cast to a graze.